Introduction to Colour Spaces - CIE Lab & LCH
This is a very basic introduction to two related colour spaces which are becoming increasingly important in the world of colour reproduction. These are among the tristimulus (three-dimensional) colour spaces developed by the C.I.E.
What is the CIE?
C.I.E.
is short for "Commission
Internationale de
l'Eclairage", which in English is the "International
Commission on
Illumination".
A professional scientific organisation founded over
90 years ago to exchange
information on "all matters relating to the science and art of
lighting".
The standards for colour spaces representing the visible
spectrum
were established in 1931, but have been revised more recently.
For those of us involved in creating colour which will be reproduced on
a printed page, it is easy to forget that there are other industries
which need to describe colour! RGB or CMYK descriptions won't be of any
use to paint or textile manufacturers! Terms such as "maroon" or "navy
blue" won't be precise enough.
There
are
many CIE colour spaces, more correctly known as models, which
serve different purposes. They are all "device independent",
unlike
RGB or CMYK colour spaces which are related to a specific device
(camera, scanner, or press, etc.) and/or material type (paper,
ink set, film
emulsion or lighting, etc.). These RGB and CMYK spaces
usually do not cover the entire
visible colour spectrum or gamut. The CIE also specify
lighting conditions.
The CIE LCH Colour Space or Colour Model.
This is possibly a little
easier to
comprehend than the Lab
colour space, with which it shares several
features. It is more correctly known as L*C*H*.
Essentially
it is in the form of a sphere. There are three axes; L* and C*
and H°.
The L* axis represents Lightness. This is vertical; from 0 which has no lightness (i.e. absolute black) at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.
The C* axis represents Chroma or "saturation". This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma or saturation.
If we take a horizontal slice through the centre, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or "Hue". This circular axis is known as H° for Hue. The units are in the form of degrees° (or angles), ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) and back to 0°.
The LCH colour model is very useful for retouching images in a colour managed workflow, using very high-end editing or scanning applications such as Fuji ColourKit. LCH is device-independent. A similar colour model is HSB or HSL for Hue, Saturation and Brightness (Lightness) which can be used in Adobe PhotoShop and other software. Technically this is "device-dependent", however it is particularly useful for editing RGB images. For example to edit a green: Adjust the Hue angle by increasing it to make it "bluish" or by reducing it to make it "yellowish"; Increase the Saturation (Chroma) to make it "cleaner"; increase the Brightness or Lightness to make it lighter. Go on give it a try!
The CIE Lab Colour Space or Colour Model.
This
is
more
correctly known as L*a*b*. Just as in LCH,
the vertical L*
axis
represents "Lightness", ranging from 0-100. The other
(horizontal) axes are
now represented by a* and b*. These are at right angles to
each
other and cross each other in the centre, which is neutral (grey, black
or white).
They
are based on the principal that a colour cannot be both red and green,
or blue and yellow. The a* axis is green
at one extremity
(represented by -a), and red at the other (+a). The b* axis
has
blue at one end (-b), and yellow (+b) at the other. The
centre of
each axis is 0. A value of 0 or very low numbers of both a* and b* will
describe
a
neutral or near neutral.
CIE Lab is extensively
used in many
industries apart from printing and photography. It's uses
include providing exact colour
specifications for
paint
(including automotive, household, etc.), dyes (including textiles,
plastics, etc.),
printing ink and paper. Nowadays it is becoming of increasing
importance in
specifying printing standards such as in ISO-12647, where it is usually
used instead of densitometry. For example "Paper Type
1" (115gsm gloss coated white, woodfree) has "Paper Shade" described as
L* 95, a* 0, b* -2. So the L*95 is very light,
the a*0 neutral, and the b*-2 very slightly "blueish". "Paper
Type 5" (115gsm uncoated yellowish offset) is described as L* 90, a* 0,
b* 9. So a darker, "yellower" paper. If you compare the different Lab
values for Type 1
& 5 you will understand the descriptions. Lab measurements
can be used to control printing, typically by monitoring a 3-colour
neutral grey mid-tone patch. It is also very useful for specifying a
"spot colour" perhaps an important "house" or "corporate" colour such
as "Coca-Cola Red". The same colour could be used for printed matter,
vans, clothing, buildings and of course tin cans.
In ICC Colour Management CIE Lab is often used as the Profile Connection Space (PCS) where it provides a "link" between two colour profiles, such as Input RGB (scanner or camera) and Output CMYK (press or inkjet). All ICC profiles contain a PCS. In an input profile the tables will convert the scanner's or camera's RGB space to the PCS (Lab). An output profile will convert the PCS (Lab) to the digital printer's or printing press's colour space (CMYK). The other PCS colour space is CIE XYZ, which is often also used by spectrophotometers to report colour, but that is beyond the scope of this article; maybe later!
For more in depth information visit the Links page of the Colour Imaging Group of the London College of Communication. www.digitalcolour.org/Links.htm
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